We have developed a simple monoclonal antibody immunoadsorption technique for isolating Factor VIII procoagulant protein (VIIIC) from commercial Factor VIII concentrates in milligram quantities. Though this material has undergone some proteolysis, it has allowed us to 1) produce heterologous precipitating antibodies and monoclonal antibodies to VIIIC, 2) describe the generation of an Mr = 92,000 dalton polypeptide in association with thrombin-induced activation of VIIIC activity and propose a model for thrombin proteolysis of VIIIC, 3) demonstrate characteristic differences between proteolytic products of VIIIC produced by activated protein C and thrombin, 4) obtain evidence for immunologic alterations in VIIIC following thrombin proteolysis and develop monoclonal antibodies which identify such changes. We intend to use these initial studies as a basis for 1) isolation of unproteolyzed "native" VIIIC; 2) further definition of proteolytic cleavage of VIIIC by thrombin and activated protein C; comparison with cleavage by plasmin, Factor Xa and Factor IXa; and isolation of VIIIC proteolytic fragments for functional and immunologic studies; 3) development of monoclonal antibodies specific for functional epitopes and proteolytic fragments of VIIIC; 4) delineation of VIIIC interactions with von Willebrand factor, platelets, and Factor IX; 5) definition of molecular abnormalities of VIIIC in hemophilia A, and combined Factor V and Factor VIII deficiency; 6) definition of proteolytic alterations of VIIIC in baboon models of thrombosis; 7) definition of proteolytic alterations of VIIIC in human subjects with disseminated intravascular coagulation or thromboembolic disease; 8) localization of VIIIC in tissues and identification of its site of synthesis.